1. Field of the Invention
This invention relates to the preparation of N-(posphonomethyl)glycine by the hydrogenation of mixtures produced by the reaction of glycolic acid and oxygen in an aqueous solution containing aminomethylphosphonic acid (AMPA) and the enzymes glycolate oxidase ((S)-2-hydroxy-acid oxidase, EC 1.1.3.15) and catalase (EC 1.11.1.6). N-(Phosphonomethyl)glycine is a broad-spectrum, post-emergent herbicide useful in controlling the growth of a wide variety of plants.
2. Description of the Related Art
Numerous methods are known for preparing N-(phosphonomethyl)glycine from aminomethylphosphonic acid and glyoxylic acid. One such method, described in Rogers et al., European Patent Application 186,648, involves condensation of glyoxylic acid or a salt thereof with aminomethylphosphonic acid or a salt thereof to form an intermediate product, generally regarded as an aldimine (Schiff base), which without isolation is reduced, as by catalytic hydrogenation, to N-(phosphonomethyl)glycine. A second method, described in Gaertner, U.S. Pat. No. 4,094,928, isolates these same intermediate carbonylaldiminomethanephosphonates by the reaction of glyoxylic acid esters with aminomethylphosphonate esters in a non-aqueous solvent; after azeotropic distillation of water and removal of the solvent, the carbonylaldiminomethanephosphonate ester is reduced and the ester groups hydrolyzed to produce N-(phosphonomethyl)glycine.
The above routes to N-(phosphonomethyl)glycine suffer in that glyoxylic acid is a rather costly starting material, and other less expensive routes to the desired material are practiced. Existing methods for the preparation of glyoxylic acid, such as hydrolysis of a dihaloacetic acid, electrolytic reduction of oxalic acid, oxidation of glyoxal, catalytic oxidation of ethylene or acetaldehyde, and ozonolysis of maleic acid, its esters or anhydride, present one or more difficulties in practice, e.g. costly separation/purification steps, low yields, or large waste streams. The method described in Gaertner is also disadvantageous in that it requires several additional steps (with corresponding losses in yield), and the unnecessary isolation of an intermediate.
Another method for the synthesis of N-(phosphonomethyl)glycine, disclosed in Kleiner, U.S. Pat. No. 4,670,191, comprises the reaction of aminomethylphosphonic acid or a salt thereof with about two molar equivalents of glyoxylic acid in aqueous medium. The excess glyoxylic acid evidently functions as a reducing agent, converting an intermediate glyoxylic acid-aminomethylphosphonic acid reaction product to the desired N-(phosphonomethyl)glycine, and is itself oxidized to one or more by-products, including CO.sub.2. Similarly, Fields et al., in U.S. Pat. No. 4,851,159 prepare N-(phosphonomethyl)glycine by heating an N-acylaminomethylphosphonic acid with glyoxylic acid or a derivative thereof. The mole ratio of the glyoxylic to the N-acylamino component is preferably 2 to 1; otherwise at smaller ratios the yield suffers.
The Kleiner and Fields et al. processes entail the disadvantages of not only employing relatively expensive glyoxylic acid but of employing it as a sacrificial reductant (ca. one mole of glyoxylate employed as reductant for every mole of N-(phosphonomethyl)glycine produced) as well as the condensing agent for the amino-(or N-acylamino) methylphosphonic acid.